(1) Field of the Invention
The present invention generally relates to an engine control apparatus for an internal combustion engine, and more particularly to an apparatus for controlling operating parameters of a multicylinder internal combustion engine, the operating parameters including fuel injection correction factors or exhaust gas recirculation factors, controlling of the operating parameters being performed, during a fuel injection correction process with respect to the cylinders, the torque variation being so adjusted to agree with a target torque variation.
(2) Description of the Related Art
A conventional fuel injection control device is known, for controlling fuel injection quantities of a multicylinder internal combustion engine. Japanese Laid-Open Patent Application No. 1-271634, for example, discloses a fuel injection quantity control device for a multicylinder internal combustion engine. In this device, a combustion pressure sensor for detecting a combustion pressure is mounted in one of a plurality of cylinders of the engine. A cycle-by-cycle torque variation is calculated on the basis of a signal indicative of the combustion pressure being output by the pressure sensor each time a combustion/expansion stroke occurs in the cylinder. A feedback process of correcting a fuel injection quantity is performed by making an air-fuel mixture in the engine as lean as possible (or, making the air-fuel ratio as close to its lean-side limit as possible), in such a way that the calculated torque variation is adjusted so as to be approximately equal to a given target torque variation.
The fuel injection quantity control process is performed by this conventional device so as to make the cycle-by-cycle torque variation approach its lean-side limit; this fuel injection quantity control is called hereinafter a lean-limit control. By performing such a lean-limit control process, the fuel consumption can be improved and the quantity of nitrogen oxides NOx in the exhaust gas can be effectively reduced.
In the above conventional device, in order to avoid increase of manufacturing costs as well as complexity of the control method, a single combustion pressure sensor is mounted on only one cylinder among the plurality of cylinders. But, a signal output by this combustion pressure sensor cannot be used directly in the fuel injection quantity control as the representative pressure of all the combustion pressures produced in the cylinders. The fuel injection quantities to be injected into the cylinders inherently have various limits at which a misfire may occur if the fuel injection quantity exceeds the limit, and there are variations of the quantity of recirculated exhaust gas due to the differences of valve clearances in the cylinders. Thus, if the signal output by the combustion pressure sensor is used directly as the representative combustion pressure, a misfire may occur in any cylinder and the torque variations in the plurality of cylinders become greater.
In the above conventional device, therefore, differences in the generated torque between the cylinders are detected from the time required for a combustion and expansion stroke to take place in each of the cylinders, and an inter-cylinder correction process is performed for eliminating the torque differences and bringing the quantities of torque generated in the cylinders into agreement with one another by correcting an air-fuel ratio in each of the cylinders. After this inter-cylinder correction process has been completed, the above described lean limit control process is started.
However, in the conventional device there is a problem in that it is necessary to meet prescribed operating conditions of the engine before correction factors used for the inter-cylinder correction process are calculated, and a considerable time period is required for the inter-cylinder correction process to be completed. For calculating the correction factors, data must be collected more than a prescribed number of repetitions not only when each cylinder is subjected to a combustion/expansion stroke but also when each cylinder is in a fuel cut mode. Collecting of the data when each cylinder is in the fuel cut mode (not in the combustion and expansion stroke) is required for eliminating measuring errors in the data. But, when the engine is operating in a certain driving pattern, the operating condition of the engine in the fuel cut mode is only occasionally satisfied. Therefore, the conventional device in such a case requires a considerable time period to elapse until the inter-cylinder correction process is completed.
In the above described case, although the engine is in a suitable operating condition (warm-up condition, engine speed, load, etc.) for the lean limit control step to be performed for efficient fuel consumption, the conventional device cannot perform the lean limit control procedure for adjusting the air-fuel ratio to its lean side limit, while the inter-cylinder correction process is being performed. If the air-fuel ratio feedback control procedure is performed, for converging the ratio toward the stoichiometric value, the fuel consumption, when the inter-cylinder correction process has not been completed, deteriorates.